Control of mRNA translation plays a key role in temporal and spatial regulation of gene expression during development. Translational regulation of mRNAs involved in a variety of developmental processes, including oocyte polarization, cell cycle regulation, embryonic patterning and neuronal morphogenesis, has been shown to depend on sequences found within their 3' untranslated regions (3'UTRs). Ongoing identification of proteins that interact with these 3'UTR regulatory elements reveals that mRNAs often employ multiple mechanisms to regulate translation, but how these mechanisms are coordinated is poorly understood. The abundance of RNA-binding proteins in metazoan genomes suggests that these and other mechanisms of post-transcriptional control are widespread. The proposed research aims to elucidate translational control mechanisms underlying developmental processes, using the Drosophila translational repressor Nanos (Nos) as a model. Nos, together with its partner Pumilio (Pum), directs abdominal and germline development during early embryogenesis and functions later during larval development for neuronal morphogenesis. Nos provides a unique opportunity for investigating post-transcriptional regulation of developmental events because translation of nos mRNA is itself highly regulated. Translational repression of nos mRNA is mediated by a bipartite translational control element (TCE) in the nos 3'UTR, comprising two stem-loops that function at different developmental stages via distinct binding factors. Aims 1 and 2 focus on TCE-mediated repression of nos mRNA to investigate how such RNA-protein interactions effect multiple layers of translational control: Aim 1 proposes transgenic assays and biochemical characterization of translationally repressed nos mRNA-protein complexes to determine how the TCE and its regulatory factors impact the translational apparatus while Aim 2 dissects the specific contribution of one factor, Glorund. Recent studies highlighting the relevance of translational control to neuronal morphogenesis motivate new directions in Aims 3 and 4 to investigate the roles of Nos, Pum, and other RNA-binding proteins: In Aim 3, live imaging and RNA target isolation approaches will be used to elucidate the role Nos/Pum- mediated regulation in dendrite morphogenesis; Aim 4 proposes an RNAi screen to uncover the underlying contribution of post-transcriptional gene regulation to neuronal development. These studies will lead to an understanding of how 3'UTR sequences and RNA-binding proteins regulate translation during development. More generally, they will shed light on mechanisms by which RNA- protein interactions provide the highly selective control of basic cellular processes needed for development, growth, and differentiation and how the disruption of these process may lead to diseases like cancer or neurological dysfunction. PUBLIC HEALTH RELEVANCE: Control of mRNA translation enables cells to alter protein synthesis rapidly and reversibly in response to external cues. Cell cycle mediators, oncogenes, growth factors, and cytokines have been shown to be translationally regulated and defects that alter levels of proteins that control translation are associated with tumorigenesis and disruption of the immune response. By investigating translational regulatory factors and the mechanisms by which they function, the proposed work will provide fundamental insight into how these factors control development, growth and differentiation, and how the disruption of translational control can result in diseases such as cancer.